1. Field of the Invention
The present invention relates to a lens actuator for use in a recording/reproduction pick-up of various types of optical information recording/reproduction apparatuses, such as an optical video disk player, a compact digital audio disk player, and an optical data memory device.
2. Related Background Art
In the above-described optical information recording/reproduction apparatuses, recording and/or reproduction of information is normally conducted by converting a light beam emitted from a light source such as a semiconductor laser onto an optical recording medium by means of an objective lens. Furthermore, focusing control and tracking control are performed to accurately focus the light beam on a track of the recording medium by moving the objective lens in a direction of an optical axis thereof and a direction perpendicular to that direction. Such movements of the objective lens are conducted by means of a lens actuator.
FIGS. 1A and 1B are respectively schematic plan and side elevational views of a conventional lens actuator. In this lens actuator, an objective lens 101 is retained on a substantially cylindrical bobbin 102, which is rotatably and slidably supported by a supporting shaft 103. On the peripheral side surface of the bobbin 102 are provided a focusing coil 105 and a tracking coil 104 to which a magnetic field is applied by means of a yoke consisting of a magnet and a ferromagnetic member which are not shown. Conduction of currents in these coils generate a force which rotates the bobbin 102 and a force which slides the bobbin 102 along the supporting shaft 103. A reference numeral 106 denotes a prism mirror for bending a bundle of light, and a reference numeral 107 denotes a counter weight for the objective lens 101.
Such a lens actuator has been disclosed by, for example, U.S. Pat. Nos. 4,473,274 and 4,571,026. Japanese Patent Laid-Open No. 62-54837 discloses a lens actuator which has the same configuration as that of the above-described lens actuator and in which the tracking coil is mounted on the bobbin on the side opposed to the objective lens with respect the supporting shaft.
FIGS. 2A and 2B are respectively schematic plan and side elevational views of another example of the conventional lens actuator. In this lens actuator, an objective lens 111 is retained on an elliptical bobbin 108 made of a resin, which is rotatably and slidably supported by a supporting shaft 113 that passes the center of the bobbin 108. A tracking coil 109 is provided on the side surface of the bobbin 108, and a focusing coil 110 is provided on the lower portion of the bobbin 108. A magnetic field is applied to these coils 109 and 110 by means of a magnet (not shown). Conduction of currents in the coils 109 and 110 causes rotation and sliding of the bobbin 108. A counter weight 117 is mounted on the bobbin 108 on the side opposed to the objective lens 111 with respect to the supporting shaft 113. A reference numeral 116 denotes a prism mirror.
The aforementioned conventional lens actuators include the counter weight which is not associated with the driving of a lens. Provision of the counter weight prevents reduction in the weight and size of the lens actuator. However, if no counter weight is provided on the bobbin, a force which slants the bobbin will be exerted due to the weight of the objective lens, thereby generating a resistance between the bobbin and the supporting shaft and thus causing such a problem that the bobbin can not be smoothly slid.